Communication system



Feb. 27, 1962 w. c. DILLoN ET AL 3,023,276

COMMUNICATION SYSTEM Filed Aug. 23, 1957 2 sheets-sheet 1 Feb. 27, 1962 w. c. DILLON ETAL 3,023,276

COMMUNICATION SYSTEM Filed Aug. 23, 1957 2 Sheets-Sheet-2 2 Ama/Pme a 35 0 gj l 29 SP'HKE'R United States Patent O 3,023,276 COMMUNICATION SYSTEM William C. Dillon, Van Nuys, Calif., and Clarence H.

Kehm, Chicago, Ill., assignors to W. C. Dillon & Company, Inc., a corporation of Illinois Filed Aug. 23, 1957, Ser. No. 679,840 3 Claims. (Cl. 179-15) This invention is concerned with a telephone system and more particularly with a telephone system utilizing modulated carrier waves transmitted over a suitable transmission line.

Prior carrier wave telephone systems, as exemplified in Dillon and Kehm Patents 2,535,906 and 2,604,544 and vin copending Dillon and Kehm application, Serial No.

409,888, filed February 12, 1954, now abandoned, have provided a separate carrier frequency for each subscriber unit. It has been found that the center frequencies of adjacent carrier channels must be separated by at least 7,150 cycles and should be harmonically related in order to prevent objectionable inter-ference between channels. As the carrier frequency is increased the losses incurred on a cable or open Wire line also increase so that carrier systems are limited inthe number of subscribers that may be handled by the nature o-f the lin-e and the amount of carrier power available at the central station.

It is a principal object of this invention to provide a novel system which permits maximum economical utilization of the transmission line and central station equip ment of a carrier telephone system.

connected to the transmission line, each including circuitry responsive to the frequency of the carrier signal.

Another feature is the provision in such a system of frequency sensitive indicating means at each of said subscriber units and a plurality of ringing signal generators each operating at a different frequency, together with means for independently connecting the ringing signal generators to modulate the carrier signal and actuate the indicating means at a selected subscriber unit.

A further feature is that thevtelephone system includes a plurality of sources of carrier signal and a plurality of subscriber units, at least two units being responsive to each of the carrier signals.

Another feature is that the central station includes a source of carrier signal and a source of direct current, and subscriber units connected to the transmission line include units responsive to the carrier signal and other units responsive to direct current. Still a further feature is that the subscriber units responsive to the carrier signal include band pass filters while the subscriber units responsive to the direct current include low pass lters.

Further features and advantages will readily be apparent from the following specication and from the drawings in which:

FIGURE 1 is a block diagram of a system embodying the invention;

FIGURE 2 is a block diagram of the carrier subscriber unit; and

areas where a single transmission line may extend for ice many miles over a large area to serve a relatively small number of subscribers. It is economically impractical to install a separate transmission line system for each small group of subscribers or to install amplifiers at various points along the line as was necessary with prior systems.

The present invention obviates this difficulty by providing a system in which several carrier subscriber units operate on each carrier frequency and with which subscriber units operating on direct current may be connected to the transmission line along with the carrier units- Turning now to the drawings, in FIGURE 1 a system embodying the invention is illustrated in block form. Many of the details of the circuitry are not shown in detail herein and reference may be had to the aforementioned patents and pending application where specic circuits are disclosed. The telephone system includes three basic elements: a central station 10, a transmission line 11, here shown as two wires 11aand 11b, and a plurality of subscriber units 12. All of the elements shown in FIG- URE l and lying to the left of broken line 13 are preferably located at and form a part off the central station 10. Certain equipment is necessary for each carrier channel operating in the system. For carrier channel No. 1, a generator 15 produces a carrier wave of the desired frequency and suitable power for the transmission line 11. The generator 15 is connected through a line filter 16, which is esssentially a narrow band pass filter, and line phase shifter 17 to the transmission line 11. Carrier generator 15 is also coupled, through line iilter 16, with a carrier terminal unit 18. The terminal unit 18 includes a wide band pass ilter which will pass the carrier frequency together with suitable side bands -for handling the desired audio information, a phase shift network and a modulator-demodulator network in which outgoing rnessages modulate the carrier while incoming messages are demodulated. The audio infomation from carrier terminal unit 18 is coupled through wires 19 to the telephone exchange 20 which, of course, may be either dial or manual. Connections may be made from the exchange 20 to other carrier channels on the same lineor other lines, to direct current telephones or .to other central oices, as indicated.

Apparatus comparable to that provided for the first carrier channel is provided for other carrier channels. As shown in the drawing, the equipment for channel No. 2 includes generator 25, line lilter 26, line phase shifter 27 and carrier terminal unit 28. While only two carrier channels lare illustrated, it will be understood that in practice a large number of carriers may be used on a single line, limited only by the nature of the line and the amount of power available.

Connected to the transmission line 11 are a plurality of carrier subscriber units 30, 31, 32 and 33. The subscriber units `are essentially identical, except for the frequencies to which they respond, and the basic components are illustrated in FIGURE 2. The carrier subscriber unit is connected to the transmission line 11 through a band pass filter 35 and a modulator-demodulator unit 36. A receiver 37 and microphone 38 may be combined in a suitable hand set 39.

incoming modulated signals are coupled through band pass filter 35 to modulator-demodulator 36 in which the audio information is removed from the carrier. The

audio signal is amplified by amplifier 40 and coupled to receiver 37. When the person at the subscriber unit speaks, the audio is picked up by the microphone, amplified by amplifier 41 and coupled to modulator-deniodulator 36 where it modulates the carrier wave. A frequency sensitive speaker I42. is connected to the output of amplifier 40 and, as will appear, responds to a suitable ringing signal provided at the central station to indicate an incoming call for the subscriber station.

Returning to a consideration of FIGURE l, two carrier generators and 25 are shown connected to transmission line 11. It will be assumed that of the four carrier subscriber units 30, 31, 32 and 33, two are tuned to the frequency of carrier No. l, while the other two are tuned to the frequency of carrier No. 2. To permit signalling ofthe subscriber units when they are being called, a frequency sensitive indicating means is provided at each unit. In Vthe preferred .form of the invention illustrated, the indicating means is a frequency sensitive loud speaker 42, FIGURE 2. The frequency sensitive loud speaker is preferably mechanically resonant at an audio frequency and is sufficiently selective that it has very little output at frequencies other than the resonant frequency. The loud speaker at each of the subscriber units operating on the same carrier frequency-channel is designed to respond to 'a different audio frequency. For example the speaker at one unit may be designed to respond to an audio signal of 400 cycles, that at a second unit to a frequency of 600 cycles, and that at a third to a frequency oflOOO cycles. The only requirement is that the frequencies be suciently separated so that only the desired loud speaker is actuated.

Inorder to actuate the .indicating 'loud .speaker 42, and its 'counterpart 'at the other subscriber units, specialringing generators are :provided at the central station. In FIG- URE l, two vringing generatorsv 45 and 46 are shown. With the subscriber units described above, the ringing generator 45 has a frequency of 400 cycles per second while the ringing generator 46 has a frequency of 600 cycles per second. Switches 47 and 4S, associated with vringing generator '45 and switches 49 and S0, associated with ringing generator 46,'are provided for selectively connecting the ringing generators to moduate the'carrier signals. Thus, if a call is received for the subscriber unit on channel No. vl and having local indicating means responsive vto the ringing `'frequency of ringing generator 45,'switch 47 is closed, either manually by an operator or automatically in response to the selection of dial equipment, and the carrier generated by generator 15 is modulated in the modulator portion of the carrier terminal kunit 18. The modulated signal is in turn received by the subscriber unit 30, demodulated and the ringing .signal coupled to the indicating device or loud speaker 42. Similarly, switch 48 would be closed to call the subscriber unit operating `on the second carrier channel having an indicating device sensitive tothe frequency of ringing generator 45. The ringing signal may, of course, 'be either a .continuous note or may be pulsed to provide a series of short signals.

in accordance with the present invention, provision `is madefor operating direct current or D.C.'telephones on the same line with the carrier phones. Normally the D C. subscriber units will be those closest to the central station iti. The use of both carrier and -D.C. subscriber units onthe same `line permits servicing a greaternumber of `subscribers with a single transmission line, than is possible with either type ofoperation alone.

A source Y55 of direct current isprovided at the central station and .connectionis made from this source through the exchange vZtland a low pass filter S6 to the transmission line 11. Thesubscriber unit telephones 57 rand 58, which are operated by direct current, are coupled to the transmission line il through low pass lters 59 and 60 respectively. A ringingsignal generator 6l is connected through'switch z'and low pass filter-66 to the transmission line for signalling subscriber units 57 and S8. In this case the subscribers are differentiated between by virtue of coded signals, utilizing the same ringing generator 61.

Low pass filters S6, 59 and 6% prevent the carrier signals from interfering with the direct current portions of the system. A filter for elements 56, 59 and 60 is illustrated in FIGURE 3 and comprises a T-network having inductive legs 65 and 66, both 32.7 millihenry and a shunt circuit including series connected inductor 67, 22.5 millihenry and capacitor 68, 0.0913 microfarad. Of course other comparable filters may be used.

While we have shown `and described certain embodiments of our invention, it is to be understood that it is capable of many modifications. Changes therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as disclosed in the appended claims.

We claim:

l. In a telephone system: a central station; a plurality of lsources of carrier signal of different `frequency at said central station; a transmission line connected Vto said central station for transmitting said carrier signals; .a plurality of subscriber units connected ,to said transmission line at spaced points remote from said central station, there being at least two subscriber units responsive to each of said carrier signals, .each of said subscriber units including Vringing signal frequency sensitive indicating ymeans with subscriber units ,responsive to the same carrier signal having ringing .signal frequency sensitive indicating means which are responsive to ringing signals of different frequencies; a plurality of lringing signal generators at said central station, each operating -at a different frequency; and means for independently connecting said ringing signal generators to `modulate one of said carrier signals, the indicating means at a selected subscriber -unit being actuated by the selected ringing signal.

2. In a telephone system: a-centralstation; a plurality lof-sources of carrier signal of different yfrequency at said central station; a transmission line connected to said cenvtral :station for transmitting vsaid carrier signals; a plustation; a band pass filter in each of said units, each having a center frequency. the same as one of said carrier signals, `there being at least two .subscriber units tuned .to each of said carrier signals; ringing .signal frequency sensitive indicating means at each of said units, the frequency sensitive indicating means in veach of the units tuned to the same carrierfrequency being sensitive to ringing signals of different frequencies; means vfor independently modulating each of said carrier signals; a. ,plurality of ringing signal generators at Vsaid central station, each operating at a different frequency; and means for independently connecting said ringing signal generators to said modulating means, the indicatingmeans ata selected subscriber unit being actuated by the selected ringing sign-al.

3. Ina telephone system: a central station; a plurality of sources of carrier signal at said central station; .a source of direct current at said central station; a transmission line connected to said central station; a plurality of carrierfsubscriber units connected to said transmission line at spaced points remote from said central station, there being at least two subscriber units responsive to each of said carrier signals, each ofisaid subscriber units including ringing signal frequency sensitive indicating means the indicating means of subscriber unit on the same carrier frequency being responsive to ringing signal of different frequencies; a plurality of ringing signal generators at said central station each operating ata different frequency; means for independently selectively connecting said ringing signal generators to vmodulate any one of said carrier signals, the indicating means Vat a selected `transmission line at spaced points remote from said spasms subscriber unit being actuated by the selected ringing References Cited in the file of this patent signal; a' plurality of subscriber units connected to said UNITED STATES PATENTS central station and'each including'c'ircuitry responsive to 2202'474 Vroom May 28, 1940 said direct current, said units also including indicating 5 245591165 Mflnaf -I July 3. 1951 means; and a ringing signal generator associated with @5604.544 Dillon et al. July 22, 1952 said source of direct current and connectable therewith 2,721,897 Schneckloth Oct. 25, 1955 to actuate the indicating means at vthe subscriber stations 2,763,726 Weller Sept. 18, 1956 responsive to direct current. 2,857,464 .Singer Oct. 21, 1958 

